In an Open Systems Interconnection (OSI) model, a data link layer manages a communication link and executes a function related thereto. Thus, the data link layer constructs a frame used in communication and defines various details for efficient transmission of the frame. Representative examples of the function performed by the data link layer are controlling an address structure and a basic flow of a frame, controlling an error, and controlling an access to a communication medium.
FIG. 1 illustrates an exemplary frame structure for data transmission in a general Ethernet system. Referring to FIG. 1, a frame for data transmission in a general Ethernet system consists of an analog preamble carrying synchronization information between a transmitter and a receiver, a data frame which is a payload of data containing information to be actually transmitted, a digital preamble indicating the start of the data frame, a control frame for communication link control, and an acknowledge (ACK) frame indicating a response to data transmission of a counterpart. With reference to the foregoing description associated with FIG. 1, a description will now be made regarding operations of transmitting and receiving each of the frames during data transmission and reception in a general Ethernet system.
FIGS. 2A through 2C are flow diagrams illustrating exemplary data transmission and reception processes in a general Ethernet system.
FIG. 2A illustrates a data flow between a sender and a receiver in data transmission and reception processes in a general Ethernet system. As illustrated in FIG. 2A, the sender transmits a control frame to the receiver. After receiving the control frame, the receiver transmits an ACK frame to the sender in order to respond to the received control frame, thereby establishing a link between the sender and the receiver. The sender transmits a data frame to the receiver to initiate actual data communication by using the established link, and the receiver transmits an ACK frame to respond to the received data frame, thereby performing data communication. Each of the transmitted control frame and ACK frame, and the data frame transmitted after establishment of the link and a response frame thereto, i.e., the ACK frame, is transmitted together with a digital preamble behind the digital preamble.
FIG. 2B illustrates a flow of frames transmitted and received in a data transmission process in a general Ethernet system. For communication between a sender and a receiver, a data link is constructed as will be described hereinafter. As described with reference to FIG. 2A, once the sender transmits a digital preamble and a control frame to the receiver for link establishment, the receiver, after receiving the control frame, transmits the digital preamble and an ACK frame to the sender to inform the sender that the link has been established. The sender transmits the digital preamble and a data frame through the established link, and the receiver transmits the digital preamble and an ACK frame to the sender to respond to the received data. In this way, link establishment and data transmission are performed.
FIG. 2C illustrates a data flow of a part 210 indicated by a dotted line in the data transmission process illustrated in FIG. 2B. As illustrated in FIG. 2C, in the data transmission process of a general Ethernet system, in order to maintain synchronization between the sender and the receiver, the sender and the receiver transmit analog preambles to their counterparts during non-transmission of data as well as during transmission of data from the sender to the receiver or from the receiver to the sender.
When a communication protocol type as described above is applied to a visible light communication system, a visible light transmitter and a visible light receiver have to continuously maintain outputs of visible light signals in an ‘ON’ state due to the analog preambles. Since the output signals of the transmitter and the receiver are in the ‘ON’ state at all times, any change cannot be made to the visibility of communication and information differences cannot be checked visibly. Moreover, since this type of protocol is suitable on a cable without taking account of power, outputs of the transmitter and the receiver are maintained in the ‘ON’ state at all times, resulting in unnecessary power consumption.
Accordingly, there is a need for a visible light communication system capable of executing a function provided in a conventional data link layer, providing visibility to a communication channel, securing visibility, and adjusting the visibility in a communication link, and a communication protocol for the visible light communication system.